GASIFICATION

Solid and liquid carbonaceous materials can be gasified by reaction with steam, or oxygen, or mixtures of both, to make a gas containing hydrogen, carbon oxides and methane that is suitable for use as a fuel gas, or for chemical synthesis. If air is used for gasification, the nitrogen present dilutes the product gas. The composition of the gas varies according to the reactants, e.g., oil, or coal, the process conditions, and the type of reactor used.

Coal Gasification

For the gasification of coal, three main types of reactor are used: moving bed, fluidized bed and entrained flow reactors, but the basic chemical reactions are the same. Gasification involves the combustion of some of the carbon in the coal with oxygen

(1)

This provides the heat required for the endothermic reactions between steam and carbon dioxide with the remaining carbon

(2)

(3)

In moving bed reactors, lump coal is charged to the top of a fuel bed and it is gasified as it moves down the bed countercurrent to the gasifying medium, a mixture of steam and oxygen, that is introduced into the bottom of the fuel bed. The oxygen is completely consumed by reaction with residual carbon in the ash, which is then discharged from the reactor. As the hot gases from the combustion zone move upwards through the bed, steam and carbon dioxide are decomposed by reactions (2) and (3) until the temperature falls to about 1200°C, when the gasification rate becomes negligible and equilibrium is approached in the water gas shift reaction (4)

(4)

At the top of the fuel bed, the hot gases pyrolyze and dry the incoming coal and leave the reactor, together with tar, oil, etc, at temperatures of 500°C-600°C. Methane is formed in the top of the fuel bed by the reaction of hydrogen with the coal substance

(5)

The Lurgi gasifier, developed in the 1930s for the gasification of lignite, is the best example of a moving bed reactor. It produces a gas containing about 12% methane, 16% carbon monoxide, 35% hydrogen and 30% carbon dioxide, by volume, on a dry basis.

In fluidized bed reactors, the coal is crashed and charged to a bed of partly gasified coal, that is fluidized by the gasifying medium (steam and oxygen). The good heat and mass transfer that occurs enables most of the carbon in reactive coals, such as lignite, to be gasified at temperatures below the fusion point of the ash (1100°C). The gas produced contains about 80% of carbon monoxide and hydrogen with a small amount of methane (2%) and very little tar. The Winkler process, developed in the 1920s is still in commercial use at some locations.

Entrained flow reactors operate at high temperatures (1500°C) and high pressures with short residence times; the powdered-coal is entrained and gasified in cocurrent flow by the gasifying medium. Ash in the coal forms a liquid slag. The product gas, which contains mostly carbon monoxide and hydrogen and no tar or methane, leaves at the reaction temperature, and large heat recovery systems are required. Gasifiers developed by Shell and Texaco are in commercial use. In the former, dry pulverized coal is burned in oxygen, whereas, in the latter a slurry of coal in water is burned in oxygen.

Oil Gasification

Heavy and residual oils are gasified commercially using partial oxidation processes; the oil is atomized into a stream of steam and oxygen, the amount of oxygen being insufficient for complete combustion. The process is very similar to the gasification of coal in an entrained flow reactor. Indeed, the Shell and Texaco partial oxidation processes for the gasification of oil were the basis for the design of their coal gasification reactors. A typical gas composition obtained from heavy oil is 45% carbon monoxide, 45% hydrogen and 4% carbon dioxide.

The most effective way of gasifying light oils that can be purified to reduce sulfur, is by reaction with steam over a nickel catalyst, when the following reactions occur

(6)

(7)

(8)

Reactions (7) and (8) are close to equilibrium at the outlet of the catalytic reactor, the gas composition being largely determined by the temperature and pressure that prevails. At temperatures below 500°C the formation of methane is favored at the expense of hydrogen. Thus a distillate oil gasified in an adiabatic catalytic reactor with an outlet temperature of 500°C will produce a gas that contains 60% methane and 17% hydrogen, the overall reaction being exothermic.

At temperatures above 500°C, more hydrogen and less methane is produced and the reactions become strongly endothermic, the process usually being carried out in an externally fired tubular reformer with the catalyst contained within the heated tubes. At an exit temperature of 850°C the gas contains 53% hydrogen and 17% methane. Reforming processes of this type are widely used in the petrochemical industry.